Airplanes often have an on-board gas turbine engine referred to as an auxiliary power unit that provides electrical power and compressed air to various systems throughout the airplane. When the airplane is on the ground, the auxiliary power unit is the main source of power to drive the environmental control systems, air driven hydraulic pumps, and the starters for the engines. Auxiliary power units may also provide pneumatic and electric power during flight.
FIG. 1 shows a typical Auxiliary Power Unit (APU) generally denoted by the reference numeral 10. The APU 10 includes in flow series arrangement an air inlet 14, a compressor 16, a bleed port 18 for providing compressed air to the aircraft, a combustor 20 having a primary fuel nozzle 22 and a secondary fuel nozzle 24, a turbine 26 and a gas exhaust 28. Of the two nozzles 22 and 24, only the primary nozzle 22 operates during the initial stages of a startup. The compressor 16 and the turbine 26 are mounted for rotation on a shaft 30 which extends to a gearbox 32.
Mounted to the gearbox 32 is a fuel control unit 40 in fluid communication with a fuel source, (not shown), aboard the aircraft. Preferably, the fuel control unit 40 is a single stage, electromechanical fuel metering valve of the type which is well known in the art. The fuel control unit 40 includes an electrically operated torque motor 42 which has a known and repeatable relationship with a power signal from an electronic control unit (ECU) 80 which may be analog or digital. The motor 42 is directly coupled to a metering valve, not shown, and controls the valve position so that a known flow area in the metering valve corresponds to a known power signal from the ECU 80. The ECU 80 communicates with the torque motor 42 through a bi-directional circuit such as an H-bridge, and commands the motor 42 to open the valve, when an increase in fuel flow is needed, and to close the valve when a decrease in fuel is required. The ECU 80 also controls the speed at which the torque motor opens and closes the valve. From the fuel control unit 40 metered fuel flows through a conduit 46 to a flow divider 50. From the flow divider 50 the stream of fuel splits with a portion flowing through a conduit 52 which leads to the primary fuel nozzle 22 and the remainder through a conduit 54 to a secondary fuel nozzle 24.
One type of bi-directional circuit used between the ECU 80 and the torque motor 42 is the conventional H-bridge. This configuration typically uses one power supply and four power switches (i.e. power transistors) to produce the desired current flow through a load. The load is the device to which power is delivered, (i.e. the torque motor 42). It can be resistive, or inductive. The advantage of the H-bridge configuration is that it uses a single power source (i.e. the ECU 80). Referring to FIG. 2, the current to the load is controlled by means of the four power switches. During one half cycle, current flows from switch A through the load to switch D. During the other half cycle, current flows from switch B through the load to switch C.
There are, however, several disadvantages to using an H-bridge. The H-bridge requires four power switching devices for each load. Since power switching devices are usually the most expensive components of a driver, these types of switches drive up the cost of the ECU 80 in which they are mounted. There is also a size limitation. With four power switching devices, the power driver might grow beyond the size limitations of the overall design. Another disadvantage is that during any half cycle power is always being dissipated in two power switching devices, either A and D or B and C.
Accordingly, there is a need for bi-directional current circuit that has fewer switches and thus takes up less board area, costs less, dissipates less power and has fewer parts than an H-bridge.